1. Field of the Invention
The present invention relates to a blood-pressure measuring apparatus that measures a blood pressure with an inflatable cuff.
2. Related Art Statement
There is widely known a blood-pressure measuring apparatus that includes an inflatable cuff adapted to be worn on a body portion of a living subject and measures a blood pressure of the subject by slowly changing a pressing pressure of the cuff. The blood-pressure measuring apparatus is operated according to any of various known blood-pressure measuring methods including a microphone method, an oscillometric method, and an ultrasonic Doppler method. In many cases, the pressing pressure of the cuff is slowly decreased to measure a blood pressure but, in some cases, the pressing pressure is slowly increased.
In a conventional blood-pressure measuring apparatus that measures a blood pressure of a living subject with an inflatable cuff, first, a pressing pressure of the cuff is increased up to a target pressure that is estimated to be sufficiently higher than a systolic blood pressure of the subject, so that an artery located under the cuff can be completely occluded. If not, the systolic blood pressure of the subject cannot be measured. Thus, a blood-pressure measurement causes a living subject to feel some pain. In particular, in the case where a blood pressure is measured using an inflatable cuff adapted to be worn on an ankle of a living subject, the subject feels more pain because of having a higher systolic blood pressure at the ankle.
It is therefore an object of the present invention to provide a blood-pressure measuring apparatus which can measure a systolic blood pressure of a living subject without causing the subject to feel pain.
The above object has been achieved by the present invention according to which there is provided an apparatus for measuring a blood pressure of a living subject, comprising an inflatable cuff which is adapted to be worn on a body portion of the subject and which applies a pressure to the body portion of the subject; a cuff-pressure changing device which changes the pressure of the cuff between a first pressure lower than a systolic blood pressure of the body portion of the subject and higher than a mean blood pressure of the body portion, and a second pressure lower than a diastolic blood pressure of the body portion; a blood-pressure determining device which determines a diastolic blood pressure and a mean blood pressure of the subject based on a heartbeat-synchronous signal which is detected from the subject while the cuff-pressure changing device changes the pressure of the cuff; a cuff-pulse-wave detecting device which detects a cuff pulse wave as an oscillation of the pressure of the cuff; a pulse-wave-magnitude determining means for operating the cuff-pressure changing device to maintain the pressure of the cuff at a third pressure lower than the mean blood pressure of the body portion of the subject, and determining a minimal magnitude, a mean magnitude, and a maximal magnitude of the cuff pulse wave detected by the cuff-pulse-wave detecting device in a state in which the pressure of the cuff is maintained at the third pressure; and a systolic-blood-pressure determining means for determining a systolic blood pressure of the subject, based on the diastolic and mean blood pressure determined by the blood-pressure determining device and the minimal, mean, and maximal magnitudes of the cuff pulse wave determined by the pulse-wave-magnitude determining means, according to a fact that the minimal, mean, and maximal magnitudes of the cuff pulse wave correspond to the diastolic, mean, and systolic blood pressure of the subject, respectively.
According to this invention, the cuff-pressure changing device slowly changes the pressure of the cuff between a pressure lower than a systolic blood pressure of the body portion of the subject and higher than a mean blood pressure of the body portion, and a pressure lower than a diastolic blood pressure of the body portion, and the blood-pressure determining device determines a diastolic blood pressure and a mean blood pressure of the subject based on a heartbeat-synchronous signal detected from the subject during the slow changing of the cuff pressure. In addition, the pulse-wave-magnitude determining means determines a minimal magnitude, a mean magnitude, and a maximal magnitude of a cuff pulse wave detected by the cuff-pulse-wave detecting device in a state in which the cuff pressure is maintained at a pressure lower than the mean blood pressure of the body portion by the cuff-pressure changing device. And, the systolic-blood-pressure determining means determines a systolic blood pressure of the subject, based on the diastolic and mean blood pressure and the minimal, mean, and maximal magnitudes of the cuff pulse wave. Thus, the present apparatus can determine the systolic blood pressure of the subject, without having to increase the cuff pressure up to a pressure higher than the systolic blood pressure. That is, the present apparatus can determine a systolic blood pressure of a patient without having to cause the patient to feel pain.
Preferably, the cuff-pressure changing device increases the pressure of the cuff at a predetermined rate from a start pressure lower than the diastolic blood pressure of the body portion of the subject, and stops the increasing of the pressure of the cuff based on a fact that the blood-pressure determining device has determined the mean blood pressure of the subject. According to this feature, the increasing of the cuff pressure is stopped based on the fact that the blood-pressure determining device has actually determined the mean blood pressure of the subject. Therefore, the cuff pressure has only to be increased up to the lowest possible pressure to determine or estimate the systolic blood pressure. Thus, the pain the patient feels can be reduced.
Preferably, the blood-pressure measuring apparatus further comprises a blood-pressure re-determining means for increasing the pressure of the cuff up to a target pressure higher by a predetermined pressure than the systolic blood pressure determined by the systolic-blood-pressure determining means, subsequently decreasing the cuff pressure at a predetermined rate, and re-determining a systolic blood pressure of the subject based on the heartbeat-synchronous signal detected while the cuff pressure is decreased at the predetermined rate. According to this feature, the cuff pressure is increased up to the target pressure higher than the systolic blood pressure determined by the systolic-blood-pressure determining means, so that an artery located below the cuff is completely occluded and, in this state, a systolic blood pressure is re-determined. Therefore, the re-determined systolic blood pressure enjoys a high accuracy. In addition, since the target pressure is determined based on the systolic blood pressure determined by the systolic-blood-pressure determining means, the thus determined target pressure would be lower, in many cases, than a target pressure which is selected, in the conventional blood-pressure measuring apparatus, at a value sufficiently higher than an average systolic blood pressure. Thus, the pain felt by the patient undergoing the blood-pressure measurement is reduced.